Because you don't have one. We know that there is a bug on the Instapaper website that says you are still a subscriber. But your subscription has expired. His website doesn't update this status. You just need to renew your subscription.
If you want to test if you really are a subscriber or if it has expired, you can try to use the search on the Instapaper iPhone or iPad app. It uses his API too. If the search says you need to renew your subscription, you know what you need to do.